Add settings feature to convert from/to Knots/mph/Kmph

For those who have devices which transmit the speed value in Knots, it would be nice to have the option to convert in Km/h. Currently, I can see my speed on the site in Km/h but it's not real, as the vaule is expressed in Knots.

I.E: While my records says that I achieved a maximum speed of 40km/h, the real speed was 74,08 km/h (as long as 40knots is 74,08 kmph).